Six limbs adds quite a few possible advantages that you can't get with four limbs. It allows each set of limbs to be specialized for particular purposes. Consider the following body plans:
Centaur plan: Four legs for running, two arms for grasping, fighting, and using tools. Sure, we humans can manage with a bipedal stance, but it comes with a number of costs - diminished stability, back problems, and terrible childbirth. A grazing centauroid might be able to use their hands to pick or carry plants while keeping their head upright and on the lookout for predators. A predatory centaur-lion could run at full speed and use their front claws for slashing at prey repeatedly without slowing down.
Griffin plan: Wings plus a catlike body. Great for a predatory lifestyle - sure, our tetrapod birds can fly, but they have to deal with awkwardly using their feet for standing, fighting, and holding things. Of course flying griffins would need to be small, like birds, but even larger ones could control their pounces in mid-air by gliding, making them better hunters.
Pixie plan: Wings plus feet plus grasping hands. This would allow the creature to walk or run without burning the energy it would need for flight, fly when it needs to move quickly or reach high places, and use its hands to either fight or carry food back to its nest.
Jumping plan: Frogs and kangaroos are great at jumping, but pretty bad at walking normally, and hopping everywhere burns a lot of energy. Imagine a vertebrate version of a grasshopper - it could use its front four legs for running normally while keeping its back legs raised, then use its back legs for jumping when it needs the extra speed. A prey animal with this body plan could also use its back legs for delivering powerful kicks to a pursuer's face while using its front four legs for running.
Yes, six legs has some disadvantages - extra energy for growth and maintenance - but that doesn't mean they'd be useless or redundant. Using the same limbs for multiple purposes tends to mean they are less effective at their jobs than if each limb was specialized for one purpose, and more limbs means more capacity for specialization. If our early fish ancestors had six limbs for whatever reason, we'd be just as likely to re-purpose them as we would be to lose them.